Question

5. A student determined AH for the reaction below using the procedure you used in this lab but the student made mistakes. For each of the following mistakes, state whether the magnitude of the student's AH would be higher, lower, or no different from the correct AH value. Explain each answer. Mg(s) + 2HCl(aq) + MgCl2(aq) + H2(g) a. The student dropped some Mg on the lab bench while transferring it to the calorimeter after weighing it. b. The student picked up the wrong bottle and used 50 mL of 3 M HCl instead of 50 mL of 1 M HCl specified in the procedure. C. The student was in a hurry and recorded the final temperature before all the Mg had completely reacted. d. The student recorded the initial temperature, mixed the chemicals, did not put the top on the calorimeter, and then left the lab to take a phone call in the middle of the reaction. She returned after 45 minutes and recorded the final temperature.

Answer 1

Ans 5 :

a)

The magnitude of $\Delta H$ depends on the amount of reactants that react in the reaction , so if the student drops some Mg on the lab bench while transferring it to the calorimeter then the value of $\Delta H$ will decrease.

b)

If the student has used HCl of higher molarity , it indicates that higher amount of HCl is used , so the value of $\Delta H$ will increase.

c)

The value of $\Delta H$ depends on the change of temperature , if the value is recorded before all the Mg has completely reacted , the change in temperature will be less and hence value of $\Delta H$ will decrease.

d)

If the chemicals are not put on the top of calorimeter , the heat change will not be recorded and furthermore after 45 minutes the final temperature would have also been lesser , so the value of $\Delta H$ will decrease.